The glass-ceramics which contain a solid solution of β-quartz or of β-spodumene (solid solutions of β-quartz and β-spodumène) as main crystalline phase(s) are materials which are known per se, and which are obtained by heat treatment of glasses or inorganic fillers. These materials are used in various contexts and notably as a substrate for cook-tops and as fire-windows.
Transparent, opalescent, or even opaque glass-ceramics are known of various colors.
The preparation of articles made from glass-ceramic of β-quartz and/or β-spodumene classically comprise three main successive steps:                a first step of melting an inorganic glass or a filler, which is a precursor of such a glass, which is generally carried out between 1,550 and 1,650° C.,        a second step of cooling and shaping the molten glass obtained, and        a third step of crystallization or ceramming of the cooled, shaped glass, by an appropriate heat treatment.        
Upon completion of the first step of melting, it is important to remove gaseous inclusions as efficiently as possible from the molten glass mass. To this end, at least one fining agent is incorporated within it.
Arsenic oxide (As2O3) is generally used in the methods used hitherto, typically at more than 0.1% by weight and at less than 1% by weight. Antimony oxide (Sb2O3) is also used at higher contents.
In view of the toxicity of these products and of the most drastic rules in force (with reference to the safety and the protection of the environment), the incorporation of these products is sought to be minimized, even avoided, and other compounds are sought which are less toxic, even non-toxic and which are effective as fining agents.
Furthermore, for obvious reasons of economy, it is not desired to modify the operating conditions of the industrial method made use of at present. Notably, it is not desired to operate at higher temperature, which would imply spending more energy and would worsen the problems of corrosion.
Compounds other than arsenic oxide and antimony oxide are thus sought after which are effective under the same operating conditions as fining agents (compounds substituting, at least partially, advantageously totally, for said oxides) of the glass which is to be cerammed.
Notably, the use of tin oxide (SnO2) has been proposed according to prior art within the context of such research.
Patent Applications JP 111 100 229 and 11 100 230 thus describe the use of tin oxide (SnO2), alone or in combination with chlorine (Cl), at the rate of:SnO2: 0.1-2% by weightCl: 0-1% by weight.
Applications DE 19 939 787.2 and WO 02/16279 mention the use of tin oxide (SnO2), cerium oxide (CeO2), and sulfate or chlorine-containing compounds. These documents more particularly illustrate the use of tin oxide which is incorporated at less than 1% by weight. No specification on the fining performance obtained is found in said documents.
The inventor, faced with this technical problem of providing fining agents which substitute, at least partially, for As2O3 and/or Sb2O3, has studied the performances of SnO2 and has shown that this compound is not fully satisfactory alone.
The effectiveness of SnO2, as agent fining glasses precursors of glass-ceramics, increases with the amount of said SnO2 incorporated. It is thus possible to obtain good results with regard to the fining of said glasses, which good results are almost comparable to those obtained hitherto notably with As2O3, by using adequate amounts of SnO2. The incorporation of these adequate amounts, which are effective from a fining point of view, is detrimental:
firstly, due to the low solubility of SnO2 in the glass. Problems of devitrification and difficulties of implementation of the melting are observed very quickly, and
secondly, due to the reducing power of SnO2. SnO2 can reduce transition metal oxides which are present in the glass, notably vanadium oxide, and can therefore strongly influence the color of the ceramic sought after. In its presence, in the amounts which are effective for the fining of the precursor glass, the color of the final glass-ceramic is difficult to control.
It is hardly sufficient to propose using SnO2 as effective fining agent instead of the conventional fining agents (As2O3 and/or Sb2O3).
It is to the merit of the inventor to have demonstrated, in such a context, the interest of an “SnO2+Br” combination, to have observed surprisingly that such an “SnO2+Br” combination is effective as a fining agent, the SnO2 being incorporated at a low content in said combination, such that the problems set forth above are minimized, even avoided.